Abstract

Exploration for and the development of oil and gas fields in zones of abnormally high formation pressures (AHFP) require a good understanding of the origin of AHFP and the development of predictive methods. We classify the causes of AHFP into two genetic groups: (1) a syn-sedimentary group characterized by undercompaction of rocks and (2) a post-sedimentary group characterized by secondary decompaction of rocks. The choice of methods for the prediction and evaluation of AHFP should be based on the origin of abnormal pressure and the lithology of the rocks. The reservoir properties of rocks in AHFP zones suggest that large, in-place resources of hydrocarbons may be present in complex low- permeability clastic and carbonate reservoirs despite low rates of production.

Plastic deformation of reservoir rocks which result from the decrease of reservoir pressure in the course of well testing and production in AHFP zones is dependent on the origin of abnormal pressure. Careful monitoring of the critical limit of formation pressure is necessary to avoid irreversible deformation of reservoir rock. Changes in the stress field in the productive reservoir should be controlled in order to prevent the initiation of induced earthquakes. In the AHFP zones of syn-sedimentary origin, water flooding should be implemented from the start of production in order to prevent subsidence with consequent environmental damage.

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Abnormal pressures, pressures above or below hydrostatic pressures, occur on all continents in a wide range of geological conditions. According to a survey of published literature on abnormal pressures, compaction disequilibrium and hydrocarbon generation are the two most commonly cited causes of abnormally high pressure in petroleum provinces. In young (Tertiary) deltaic sequences, compaction disequilibrium is the dominant cause of abnormal pressure. In older (pre-Tertiary) lithified rocks, hydrocarbon generation, aquathermal expansion, and tectonics are most often cited as the causes of abnormal pressure.

The association of abnormal pressures with hydrocarbon accumulations is statistically significant. Within abnormally pressured reservoirs, empirical evidence indicates that the bulk of economically recoverable oil and gas occurs in reservoirs with pressure gradients less than 0.75 psi/ft (17.4 kPa/m) and there is very little production potential from reservoirs that exceed 0.85 psi/ft (19.6 kPa/m). Abnormally pressured rocks are also commonly associated with unconventional gas accumulations where the pressuring phase is gas of either a thermal or microbial origin. In underpressured, thermally mature rocks, the affected reservoirs have most often experienced a significant cooling history and probably evolved from an originally overpressured system.